engleski

Dosimetric evaluation of calculation algorithms built in Monaco treatment planning system

1. Introduction Implementation of advanced radiotherapy techniques makes high demands on performance of the treatment planning systems (TPS). Thus extensive dosimetric evaluation of respective calculation algorithms built in the TPS is required. Comprehensive verification methodology is proposed by the IAEA [1, 2]. In some recent documents [3, 4] verification of parameters especially important for the implementation of Intensity Modulated Radiotherapy (IMRT) were also proposed. In this work the results acquired using proposed methodology as well as addendums to particular methodology will be presented. Furthermore, exploration of potentials of the 2D methodology developed for the patient specific dosimetry was also performed and preliminary results will be presented. 2. Materials and Methods Dose distributions of different complexity were calculated using Monte Carlo and Colapsed Cone algorithms built in Elekta Monaco TPS. Furthermore, respective calculation methods (Dose to Water-D2W and Dose to Medium-D2M) for 6MV photon beam were used. Modeled data for both algorithms were verified in homogeneous phantoms. For this purpose extensive list of 1D measurements using ionization chambers of different volumes were performed in water phantom. Whilst modeling was confirmed, algorithms were verified using inhomogeneous (CIRS Thorax) phantom. An additional comparison was performed with calculated data using standard superposition algorithm built in Elekta XiO TPS. 3. Results Comparison of measured and calculated data in point of ‘tissue’ and low density medium show good agreement regardless of algorithm used and the calculation method. As expected Colapsed Cone algorithm shows larger discrepances when calculation point is below inhomogeneites. In high density medium deviation for both algorithms was much higher. Also, deviations of measured and values calculated with Monte Carlo algorithm as D2W and D2M , respectively, were larger and of opposite signs. To overcome this we defined minimum volume of water cylinder to be placed within high density medium. This volume allowed that D2W and D2M calculated doses differ less than 0.5% and consequently good agreement with measurements. In addition the calculations using Standard Superposition algorithm described in [5] which is part of Elekta XiO TPS were repeated and further expanded. Comparison confirmed that this algorithm shows larger discrepances in low dose regions as well as in steep gradients and also when small fields are used. Furthermore, 2D methodology used for patient specific dosimetry was used to evaluate TPS accuracy, but antrophomorphic thorax phantom was used instead of the homogeneous one. Anlayze shows that gamma criterion 3% or 3mm was not fulfilled when inhogeneities are inside treated volume. 4. Conclusion MC TPS evaluation using point dose measurements show large uncertainties within high density medium. It can be overcome by using proposed approach. Also, despite the manufacturer’s claim of 2D verification superiority over point measurements, our investigation shows otherwise when measurements in inhomogeneous phantom are considered. Good agreement in homogenous media indicates very good linac model and dose calculation of TPS in tissue-like medium. MC TPS evaluation using point dose measurements showed large uncertainties within high density medium. This problem raised question of what should be calculated to be comparable with measurements and what can be done to overcome discrepancies in D2W and D2M. Also, it points to possible limitations of the TPS algorithm. Furthermore, the concept of patient specific dosimetry using 2D detector in homogenous media might be insufficient when in-patient dose distribution should be considered.

dose calculation algorithm ; dosimetry

nije evidentirano